Several methods have been employed for forming particulate or powder-like materials into a unitary firmly compacted body of material.
Powdered metal bodies have been formed by means of pressure and heat. Such a method has also been used for forming unitary bodies from other powder or particulate materials.
A problem has specifically existed with regard to forming superconducting powders into a unitary firmly compacted body. Ceramic superconducting powders are normally prepared by proportioning the specific quantities of selected oxides. The combination is then thoroughly mixed by conventional means and then fired at elevated temperatures in suitable gaseous atmospheres. The induced solid state reaction causes the formation of the desired ceramic compositions and lattice structures.
In ceramic superconductors, the superconductivity within individual crystallites is proximity coupled to neighboring grains. Consequently, the orientation and coupling between crystallites are key factors affecting the current carrying capacity of the bulk ceramic superconductors. Voids, cracks, and grain boundaries act as weak links between crystallites and reduce the critical currents within the bulk material. Therefore, a technique which produces dense ceramics with good intergrain coupling and by which the material is formable into desired shapes to yield a required superconducting characteristic is of significant value.
At the present time several methods are used for obtaining high critical current densities in bulk superconducting materials.
One method employed is that of melt textured growth of polycrystalling material. This method is discussed in a paper included in Volume 37, No. 13, May 1, 1988, Physical Review B, S. Gin, et al, entitled: Melt-Textured Growth of Polycrystaline. This method consists of heating a bulk specimen of the high temperature material in a furnace to temperatures at which partial melting occurs. A temperature gradient is maintained in the furnace, and the superconductor is melted and recrystallised as the specimen is passed through the hot zone. Highly textured material is produced through this method and at present is shown to yield high Jc values. This method is generally limited to the processing of small length samples.
Another method is that of placing powder in a tube. This "powder in tube" method is discussed a paper 1989 Applied Physics Letters, page 2441, prepared by K. Heine, et al, entitled: High-Field Critical Current Densities. In the "powder in tube" method, mechanical deformation is used to align plate-like particles of bismuth based superconductors. The powder is loaded into a tube of silver material and the assembly is compacted by swaging, drawing or rolling. A silver sheath provides a path to shunt currents across any defects. The material is subsequently heat treated to obtain the optimum superconductor characteristics.
However, as a result of the nature of varied mechanical operation involved in the two methods discussed above, reproducing the many processing steps repeatedly during fabrication of long lengths of wires and tapes remains unsatisfactory.
Another method of compaction is that of hot extrusion. This method is discussed in an article entitled: Hot Extrusion of High-temeprature Superconducting Oxides by Uthamalingam Balachandran, et al, American Ceramic Bulletin, May 1991, page 813.
Another method is discussed in U.S. Pat. No. 5,004,722, Method of Making Superconductor Wires By Hot Isostatic Pressing After Bending.
Another compaction technique which has been employed pertains to a shock method. This method is discussed in an article entitled: Crystallographically oriented superconducting Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 by shock compaction of prealigned powder by C. L. Seaman, et al, in Applied Physics Letters 57, dated Jul. 2, 1990, page 93.
Another method of compaction is that known as an explosive method, discussed in an article entitled: Metal Matrix High-Temperature Superconductor, by L. E. Murr, et al, in Advanced Materials and Processes Inc. Metal Progress, October 1987, page 37.
These methods are limited in value because they are generally applicable only to production of small body sizes.
The application of large uniaxial static pressures at elevated temperatures is discussed in an article entitled: Densification of YBa.sub.2 Cu.sub.2 O.sub.7-8 by uniaxial pressure sintering, by S. L. Town, et al, in Cryogenics, May 1990, Volume 30.
The use of electromagnetic forming for the purpose of attachment is discussed in a paper entitled: Electromagnetic Forming, by J. Bennett and M. Plum, published in Pulsed Power Lecture Series, Lecture No. 36.
However, processing of long lengths of homogenous and high quality superconducting tapes or wires by the processes discussed above has not been realized.
It is an object of this invention to provide a method and means for producing high density bodies by the use of powder-like and/or particulate materials.
It is another object of this invention to provide a method and means for producing electrical conductors by the use of powder-like or particulate materials.
It is another object of this invention to provide a method and means for producing high quality and continuous superconducting electrical conductors such as wires and tapes.
It is another object of this invention to provide such a method which can be consistently precisely duplicated in the quality of production.
Other objects and advantages of this invention reside in the construction of parts, the combinations thereof, and the methods employed, as will become more apparent from the following description.